Stimulatory Effects of Different Cytokinin on Direct Plant Regeneration from
Cotyledon Explants in Cucumis sativus L.
Meng Zhang & Hongwen Cui
Horticulture College, Northwest Sci-Tech University of Agriculture & Forestry
Bingshan Wang & Shirong Jia
Biotechnology Institutes, Chinese Academy of Agriculture Sciences
In order to establish a regeneration system for gene
transformation, the effect of four kinds of cytokinins
(KT, BA, ZT and TDZ) were studied on direct shoot
regeneration from cotyledons of 5-day-old seedlings
of Cucumis sativus L. inbred line Jin431. ZT had a
highest efficiency among the four cytokinins, and its
regeneration frequency of 85% with multiple shoots
may be useful for gene transformation. TDZ had the
highest activation, and the optimal concentration of
TDZ was considerably lower than that of the other
three cytokinins. A new phenomenon, in which roots
were induced as well as shoots by TDZ, was found.
Keywords:
cotyledon
cucumber;
cytokinin;
organogenesis;
Introduction: Cucumber (Cucumis sativus L.) is an
important species of Cucurbitaceae and, with the
exception of watermelon, is cultivated more than any
other cucurbit in China. Recent attention has been
directed towards gene transformation technology as a
way of introducing heterogeneous genes into
cucumber without the limits of cross incompatibility
between species. There have been about 14 studies on
cucumber gene transformation by the way of
Agrobacterium-medium
or
microprojectile
bombardment from 1986. Only a few of these studies
have obtained transformants. Gene transformation
systems depend on some form of plant regeneration
technology. Most of the studies dealt with indirect
plant regeneration. A high frequency of variation in
regenerated plants occurs at the callus or somatic
embryo stage (Malepszy & Nadolska-Orczyk, 1989).
Some studies found that direct shoot regeneration
(organogenesis) from explants in cucumber were free
of morphological or physiological variation (Burza &
Malepszy, 1995; Plader et al., 1998). A system suitable
for transformation should ideally produce multiple
shoots from numerous adventitious buds with minimal
callus. This paper describes the effects of different
cytokinins on organogenesis.
Cucurbit Genetics Cooperative Report 24: 13-16 (2001)
Materials and Methods: Explant materials. Seeds of
Cucumis sativus L. inbred line Jin431 were
surface-sterilized for 10 min using a dilution of
commercial sodium hypochlorite having 3.5% active
chlorine, then rinsed three times in sterile deionized
water. The seeds were germinated on modified
Murashige & Skoog (MS, 1962) basal medium
supplemented with 3% sucrose and solidified with
0.75% agar. The pH was adjusted to 5.8 with 0.1N
NaOH prior to sterilization at 121 C for 15 min. All
plant material was incubated at 25 C with 16 hr
photoperiod at 5.0 klx provided by cool white
fluorescent lighting. Cotyledons from five-day-old
seedlings were excised and cut transversely, and the
basal segments without axillary buds were used for all
experiments. Explants were placed with the abaxial
side down on shoot induction media with the basal end
just beneath the surface of the medium.
Methods: In the preliminary experiments, it was found
that cytokinin was sufficient for shoot induction
without auxins, while auxins can increase callus
growth and decrease shoot development. In order to
investigate the influence of different cytokinins on
shoot induction and to select the optimal concentration,
shoot induction media were prepared by modifying the
MS medium above with the following concentrations
of individual cytokinins: (1) kinetin (KT)
concentrations at 0, 0.5, 1.0, 2.0, 5.0, 10.0, and 20.0
mg l-1, (2) 6-benzylaminopurine (BA) concentrations
at: 0, 0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 mg l-1, (3) zeatin
(ZT) concentrations at: 0, 0.2, 0.5, 1.0, 2.0, 5.0, and
10.0
mg
l-1,
(4)
and
thidiazuron
(N-pheny1-N’-1,2,3-thiadiazol-5yl-urea,
TDZ)
concentrations at: 0, 0.0001, 0.001, 0.005, 0.01, 0.05,
and 0.1 mg l-1. (KT, BA and ZT were obtained from
Sigma Co., while TDZ was synthesized by the
Northwest Plant Institute, Chinese Academy of
Sciences.)
Explants were cultured on shoot induction media
under the same conditions as seedlings. There were
10 replicates per treatment with four explants per
13
replicate. Shoot differentiation and induction
frequencies were investigated at the 20th day after
induction. Statistical analysis was conducted
according to a single-factor design.
Results: Protuberances could be found at the basal
region of cotyledons after 7-8 days induction when
cotykinin concentrations were suitable, and little
buds could be found after two weeks. These buds
could develop to 1-2 cm in size after 3 weeks
(Fig.1).
The effects of a variety of different cotyledons on
shoot induction, and statistical analyses of KT, BA
and ZT concentrations are shown in Table 1, while
the results of shoot induction frequencies and
statistical analysis of TDZ are shown in Table 2,
since the range of TDZ concentrations was much
lower that the other three. The results of the four
kinds of cytokinins exhibited the same tendency in
that shoot induction frequency was low and the
callus grew slowly in media with low cytokinin
concentrations, shoot induction became optimal and
the callus grew moderately with increasing
cytokinin concentrations, and shoot induction then
decreased sharply and while callus growth was
greatest with the further increases in cytokinin
concentration, although the callus grew little at the
highest concentration of cytokinin. The cytokinins
KT, BA and ZT could induce multiple shoots at the
basal region of cotyledons in the media with each
cytokinin’s optimal concentration. The number of
shoots induced per explant was usually more than
10, and 25-30 at maximum. The cytokinin TDZ
induced fewer shoots at the base of the cotyledon
than the other three cytokinins, with fewer than 3
shoot developed per explant. There was a new
phenomenon in that roots were induced as well as
shoots in the shoot induction medium with a TDZ
concentration of 0.0001 to 0.01 mg l-1 (Fig. 2). The
root induction frequency increased with decreasing
concentration.
The optimal concentration of KT was 5.0 mg l-1, of
BA was 0.5 mg l-1, of ZT was 1.0 mg l-1, and of
TDZ was 0.005 mg l-1. The cytokinin TDZ had a
higher activation on shoot induction than the other
three, and the optimal concentration of TDZ was
considerably lower. ZT had the highest efficiency
(85%) of shoot induction of the selected inbred line.
The concentration range of cytokinins on shoot
induction differed from each other. TDZ could
Cucurbit Genetics Cooperative Report 24: 13-16 (2001)
induce shoots at concentration of 0.00001 to 0.05
mg l-1 and ZT had a relative high effective at
concentration of 0.2 to 5.0 mg l-1, while KT and BA
had a relative narrow range of concentration on
shoot induction.
Discussion: Wehner and Locy (1981) studied the
direct regeneration with cotyledon and hypocotyl
from 7-day-old seedlings as explants on the MS
medium containing 1.0mgl BA and 1.0 mgl-1 NAA.
Eighty-five cultivars were tested and just 28
cultivars developed shoots with an average shoot
induction frequency of 5.5%. That is likely due to
the addition of NAA and explants from elder
seedling. In order to establish the cucumber gene
transformation system, Gambley and Dodd
(1990,1991) studied successfully the shoot direct
regeneration with three kinds of cytokinins KT, BA
and 2-ip (N6-(2-isopentenyl)adenine). They found
that concentration (4 mgl-1 or less) of the cytokinins
KT, BA and 2-ip with or without low concentration
of auxin, were all effective in producing multiple
adventitious buds at the base of cotyledons from 5-7
-day-old seedling. BA had a best efficiency in the
three kinds of cytokinins. In this studied, the similar
results were obtained. We found that the auxin
restrained the shoot development even at low
concentration in the preliminary experiments (data
not showed), and the ZT had a highest efficiency on
inducing shoots of the selected inbred line among
the four kinds of cytokinins. So this multiple
adventitious shoots regeneration technique should
be useful for transformation studies in cucumber.
We had got some of chitinase/ ß-1, 3-glucuronidase
genes and glucose oxidase gene transformants by this
direct organogenesis technique.
On the other hand, we observed a new phenomenon
that roots were induced as well as shoots were
induced. That may be due to the higher endogenesis
auxins compared with the absolute amount of TDZ.
References
1. Wehner, T.C. and R.D. Locy (1981) In vitro
adventitious shoot and root formation of
cultivars and lines of Cucumis sativus L..
HortScience 16: 757-760
2. Gambley, R.L. and W.A. Dodd (1990) An in
vitro technique for production de novo of
multiple shoots in cotyledon explants of
14
cucumber (Cucumis sativus L.) Plant Cell,
Tissue and Organ Culture 20:177-183
in the cucumber
Euphytica 103:9-15
3. Gambley, R.L. and W.A. Dodd (1991) The
influence of cotyledons in axillary and
adventitiois shoot production from cotyledonary
nodes of Cucumis sativus L. (cucumber).
Journal of Experimental Botany 42(242);
1131-1135
(Cucumis
sativus
L.).
5. Burza, W. and S. Malepszy. (1995) Direct plant
regeneration from leaf explants in cucumber
(Cucumis sativus L.) is free of stable genetic
variation. Plant Breeding 114:341-345
6. Malepszy, S. and A. Nadolska-Orczyk A. (1989)
In vitro culture of Cucumis sativus VIII.
Variation in the progeny of phenotypically not
altered R1 plants. Plant Breeding 102:66-72
4. Plader, W., S. Malepszy S., W. Burza and Z.
Rusinowski (1998) The relationship between
the regeneration system and genetic variability
Table 1. Shoot induction efficiency of three kinds of cytokinins
Concentration (mg/l)
cytokinins
0
0.2
0.5
1.0
2.0
5.0
10.0
20.0
5cdBC
20bB
20bB
50aA
15bcB
0dC
KT
0dC
BA
0dC
20cC
79aA
70abAB
55bB
8dC
0dC
ZT
0cC
45bB
60bAB
85aA
50bB
39bB
0cC
Notes:
a,b,c and d indicate significance at the 0.05 level.
A, B and C indicate significance at the 0.01 level
Cucurbit Genetics Cooperative Report 24: 13-16 (2001)
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Table 2. Shoot induction efficiency of different concentration of TDZ
Concentration
(mg/l)
Shoot induction
(%)
significance level
(0.05)
significance level
(0.01)
0.005
60
a
A
0.01
35
b
B
0.05
25
bc
BC
0.001
20
c
BC
0.0001
5
d
CD
0.1
0
d
D
0
0
d
D
Notes:
a,b,c and d indicate significance at the 0.05 level.
A, B and C indicate significance at the 0.01 level
Fig. 1 Shoots induced by cytokinin ZT.
Fig. 2 Shoot and roots induced by
cytokinin TDZ.
Cucurbit Genetics Cooperative Report 24: 13-16 (2001)
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